A relationship between sudden infant death syndrome (SIDS), sleep apnea, and upper airway infections has been reported. The present observation stresses the possible influence of phenothiazine-containing medications and the occurrence of SIDS. The drug is commonly used for the treatment of infants with nasopharyngitis in Belgium and in some other European countries. In a prospective study, 52 SIDS victims, 36 near miss infants, and 175 control infants were compared for the coexistence of nasopharyngitis and phenothiazine treatment in the days preceding death or hospitalization. The incidence of nasopharyngitis was comparable in the three groups (approximately 31%), but phenothiazines were used significantly more frequently in SIDS victims (23%) and near miss infants (22%) than in control subjects (2%). It is postulated that phenothiazines, as CNS depressors, may contribute to the occurrence of SIDS.